1. Field of the Art
The present invention relates to a field-sequential color television camera for generating three primary color image signals of an object image field by field in sequence by scanning an image sensor which can transduce sequential primary color images into video signals
2. Prior Art
In recent years, a remarkable progress of computer-related digital processing technology has enabled a huge amount of information to be processed in a relatively simple system configuration. Further, since image processing technique has been developed with a remarkable progress in the above-mentioned digital processing technology, there exists a strong demand for low-priced high-quality image sensing devices connectable to a computer.
As one of image sensing devices which can satisfy the above-mentioned demand, field-sequential color television cameras are well-known. This is because this camera has various advantages such that the structure is simple, the size is small, and the cost is relatively low as compared with a simultaneous color television camera. Further, when compared with a single-tube color television camera having a color stripe filter, this camera provides a high picture resolving power because a single image tube is used in common for three primary colors in time sharing manner.
In this field-sequential color television camera, an image sensor is exposed to a single primary color image and then scanned to obtain the single primary color image field; thereafter the image sensor is exposed to another primary color image and then scanned to obtain the succeeding primary color image field; and the above operations are repeated in sequence to obtain video signals of red (R), green (G) and blue (B) in time-sharing method.
FIG. 1 shows an example of prior-art color separating rotary filters. This filter 100 formed with four sets of three primary color filter segments R, B and G is disposed in front of an image sensor and rotated in the arrow direction A in synchronism with field scanning operation, so that three primary color images can be applied in sequence onto the image sensor.
In the prior-art field-sequential color television camera, however, since the color filter segments for the respective three different primary colors are arranged continually one after another and therefore three primary color image signals of an object image are detected continually in sequence of field, there exists a problem in that it is impossible to obtain accurate primary color image signals due to after-image (residual image) characteritics of the image sensor, that is, residual photoelectric charge caused by high speed scanning operation. In more detail with reference to FIG. 2, a primary color image signal (e.g. green field signal) 103 is mixed with a residual signal 102 of another primary color image signal (e.g. preceding red field signal) 101.
In addition, the image sensor includes capacitive response characteristics as shown in FIGS. 3A and 3B. FIG. 3A shows fall-response characteristics when light allowed to be incident upon the image sensor to obtain a video signal by scanning is turned from ON to OFF at time t=t.sub.1. This graph indicates that the residual image of the preceding field is high in signal level at the first field No. 1, and decreases gradually as the field Nos. increase after the turn-off time t=1.sub.1. In contrast with this, FIG. 3B shows rise-response characteristics when light is allowed to be incident upon the image sensor; that is, turned from OFF to ON at time t=t.sub.2. This graph indicates that the image signal level is low at the first field No. 1, but increases gradually as the field Nos. increase after the turn-on time t=t.sub.2.
In summary, in the prior-art method, color is mixed or darkened due to the after-image (residual) characteritics of the image sensor. Further, when an object image of high color saturation (e.g. monochrome) is image-detected, since the image sensor is exposed to a primary color image only intermittently, the detected signal level is low due to the rise-response characteritics of the image sensor as compared with when a white color image is image-detected (This is because in the case of white color, the image sensor is exposed to primary color images continually). Therefore, the obtained field-sequential color television signal is low in color saturation.
To overcome the above-mentioned color-turbidity problem, Japanese Patent Appl. No. 61-290147 has proposed a certain technique, which comprises color separating means for alternately providing an exposure period (during which an image sensor is exposed to a primary color image from an object image to accumulate photoelectric charge) and a shading period (during which the image sensor is shaded from the primary color image); and scanning means for scanning the image sensor upon start of the shading period to output a primary color image signal of the object image as the accumulated photoelectric charge and further upon start of the exposure period to discharge the residual photoelectric charge, in order to prevent color turbidity. In this prior-art method, however, since light allowed to be incident upon the image sensor is shaded every other field, there still exists a problem in that the obtained video signals are low in color saturation due to the afore-mentioned rise response characteristics of the image sensor, and therefore it is impossible to obtain accurate video images, although color turbidity due to residual image characteristics of the image sensor can be eliminated to some extent.